Read sensor with confined sense current

ABSTRACT

A magnetoresistive (MR) sensor having a decreased electrical profile due to a confining of the device sense current within a conductive nanoconstriction. The MR sensor includes a giant magnetoresistive (GMR) stack and a layer of high resistivity material within the GMR stack. The layer of high resistivity material includes a nanoconstriction precursor. When a punch current is applied at the nanoconstriction precursor, a conductive nanoconstriction is formed through the layer of high resistivity material at the nanoconstriction precursor.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of magnetic datastorage and retrieval systems. More particularly, the present inventionrelates to a transducing head including a current perpendicular to theplane (CPP) read sensor having a sense current-confining conductivenanoconstriction.

In a magnetic data storage and retrieval system, a magnetic recordinghead typically includes a reader portion having a magnetoresistive (MR)sensor for retrieving magnetically encoded information stored on amagnetic disc. Magnetic flux from the surface of the disc causesrotation of the magnetization vector of a sensing layer or layers of theMR sensor, which in turn causes a change in electrical resistivity ofthe MR sensor. The sensing layers are often called “free” layers, sincethe magnetization vectors of the sensing layers are free to rotate inresponse to external magnetic flux. The change in resistivity of the MRsensor can be detected by passing a current through the MR sensor andmeasuring a voltage across the MR sensor. Depending on the geometry ofthe device, the sense current may be passed in the plane (CIP) of thelayers of the device or perpendicular to the plane (CPP) of the layersof the device. External circuitry then converts the voltage informationinto an appropriate format and manipulates that information as necessaryto recover the information encoded on the disc.

The essential structure in contemporary read heads is a thin filmmultilayer containing ferromagnetic material that exhibits some type ofmagnetoresistance. Examples of magnetoresistive phenomena includeanisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), andtunneling magnetoresistance (TMR).

AMR sensors generally have a single MR layer formed of a ferromagneticmaterial. The resistance of the MR layer varies as a function of cos²α,where α is the angle formed between the magnetization vector of the MRlayer and the direction of the sense current flowing in the MR layer.

GMR sensors have a series of alternating magnetic and nonmagneticlayers. The resistance of GMR sensors varies as a function of thespin-dependent transmission of the conduction electrons between themagnetic layers separated by the nonmagnetic layer and the accompanyingspin-dependent scattering which takes place at the interface of themagnetic and nonmagnetic layers and within the magnetic layers. Theresistance of a GMR sensor depends on the relative orientations of themagnetization in consecutive magnetic layers, and varies as the cosineof the angle between the magnetization vectors of consecutive magneticlayers.

A typical GMR read sensor configuration is the GMR spin valve, in whichthe GMR read sensor is a multi-layered structure formed of a nonmagneticspacer layer positioned between a synthetic antiferromagnet (SAF) and aferromagnetic free layer, or between two ferromagnetic free layers. Inthe former case, the magnetization of the SAF is fixed, typically normalto an air bearing surface (ABS) of the GMR read sensor, while themagnetization of the free layer rotates freely in response to anexternal magnetic field. The SAF includes a reference layer and a pinnedlayer which are magnetically coupled by a coupling layer such that themagnetization direction of the reference layer is opposite to themagnetization of the pinned layer. In the latter case, themagnetizations of the two free layers rotate freely in response to anexternal magnetic field. The resistance of the GMR read sensor varies asa function of an angle formed between the magnetization direction of thefree layer and the magnetization direction of the reference layer of theSAF, or as a function of an angle formed between the magnetizationdirections of the two free layers. This multi-layered spin valveconfiguration allows for a more pronounced magnetoresistive effect, i.e.greater sensitivity and higher total change in resistance, than ispossible with anisotropic magnetoresistive (AMR) read sensors, whichgenerally consist of a single ferromagnetic layer.

TMR sensors have a configuration similar to GMR sensors, except that themagnetic layers of the sensor are separated by an insulating film thinenough to allow electron tunneling between the magnetic layers. Thetunneling probability of an electron incident on the barrier from onemagnetic layer depends on the character of the electron wave functionand the spin of the electron relative to the magnetization direction inthe other magnetic layer. As a consequence, the resistance of the TMRsensor depends on the relative orientations of the magnetization of themagnetic layers, exhibiting a minimum for a configuration in which themagnetizations of the magnetic layers are parallel and a maximum for aconfiguration in which the magnetizations of the magnetic layers areanti-parallel.

For all types of MR sensors, magnetization rotation occurs in responseto magnetic flux from the disc. As the recording density of magneticdiscs continues to increase, the width of the tracks on the disc mustdecrease, which necessitates smaller and smaller MR sensors as well. AsMR sensors become smaller in size, particularly for sensors withdimensions less than about 0.1 micrometers (μm), the sensors have thepotential to exhibit an undesirable magnetic response to applied fieldsfrom the magnetic disc. MR sensors must be designed in such a mannerthat even small sensors are free from magnetic noise and provide asignal with adequate amplitude for accurate recovery of the data writtenon the disc.

To sustain a compound annual growth rate in areal density of 60% or moreover the next few years, read widths of less than 40 nm will berequired. At these dimensions, the capability of conventionallithographic steppers and etch/strip processes to maintain adequatetargeting and sigma control is uncertain. Alternative technologies thatrelax lithographic line width requirements while hitting electrical andmagnetic width targets are desirable.

One promising technique to reduce the effective dimensions of MR sensorsis to incorporate current confining paths, or “pinholes,” in a layer orlayers of the MR stack. The current confining paths are formed such thatthey offer a path of lower resistance through which the sense currentflows. The sense current is thus confined to a smaller portion of the MRstack, thereby reducing the electrical profile of the MR sensor.Typically, these current confining paths are formed either by etching acurrent confining path into a layer or layers of the MR stack, or byincorporating a layer of granular or porous material into the MR stackhaving naturally occurring current confining paths. Both of thesetechniques for including current confining paths in an MR sensor aredescribed in, for example, patent application Pub. 2002/0051380 byKamiguchi et al. The present invention is a more controllable approachto forming current confining paths in an MR sensor which allows for anincreased magnetoresistive signal.

BRIEF SUMMARY OF THE INVENTION

The present invention is a current-perpendicular-to-plane (CPP)magnetoresistive (MR) sensor having a decreased electrical profile dueto a confining of the device sense current within a conductivenanoconstriction. The MR sensor includes a giant magnetoresistive (GMR)stack and a layer of high resistivity material within the GMR stack. Thelayer of high resistivity material includes a nanoconstrictionprecursor. When a punch current is applied at the nanoconstrictionprecursor, a conductive nanoconstriction is formed through the layer ofhigh resistivity material at the nanoconstriction precursor.

In one embodiment, the nanoconstriction precursor comprises a thinnedregion in the layer of high resistivity material. In another embodiment,the nanoconstriction precursor comprises a region in the layer of highresistivity material which has been implanted with metal ions by an ionbeam. In a further embodiment, the nanoconstriction precursor comprisesa region in the layer of high resistivity material which has beentransformed to a low resistivity material by an electron beam. In stilla further embodiment, the nanoconstriction precursor comprises a regionin the layer of high resistivity material which has been reduced to ametal via a reactive ion etch.

In all embodiments, a width of the conductive nanoconstriction isadjustable by adjusting an amplitude and duration of the punch current.Furthermore, the shape of the conductive nanoconstriction is adjustableby adjusting a thickness of the layer of high resistivity material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a magnetic read/write head andmagnetic disc taken along a plane normal to an air bearing surface ofthe read/write head.

FIG. 2 is a layer diagram of an air bearing surface (ABS) of themagnetic read/write head of FIG. 1.

FIG. 3 shows an ABS view of a typical tri-layercurrent-perpendicular-to-the-plane (CPP) GMR stack.

FIG. 4 a shows an ABS view of a tri-layer CPP MR stack according to anembodiment of the present invention.

FIG. 4 b shows a top view of a tri-layer CPP MR stack shown in FIG. 4 a.

FIG. 5 shows a perspective ABS view of a tri-layer CPP MR stackaccording to another embodiment the present invention.

DETAILED DESCRIPTION

FIG. 1 is a cross-sectional view of a magnetic read/write head 10 andmagnetic disc 12 taken along a plane normal to air bearing surface 14 ofread/write head 10. Air bearing surface 14 of magnetic read/write head10 faces disc surface 16 of magnetic disc 12. Magnetic disc 12 travelsor rotates in a direction relative to magnetic read/write head 10 asindicated by arrow A. Spacing between air bearing surface 14 and discsurface 16 is preferably minimized while avoiding contact betweenmagnetic read/write head 10 and magnetic disc 12.

A writer portion of magnetic read/write head 10 includes top pole 18,insulator 20, conductive coils 22 and bottom pole/top shield 24.Conductive coils 22 are held in place between top pole 18 and top shield24 by use of insulator 20. Conductive coils 22 are shown in FIG. 1 astwo layers of coils but may also be formed of any number of layers ofcoils as is well known in the field of magnetic read/write head design.

A reader portion of magnetic read/write head 10 includes bottom pole/topshield 24, bottom shield 28, and magnetoresistive (MR) stack 30. MRstack 30 is positioned between terminating ends of bottom pole 24 andbottom shield 28. Bottom pole/top shield 24 functions both as a shieldand as a shared pole for use in conjunction with top pole 18.

FIG. 2 is a layer diagram of air bearing surface 14 of magneticread/write head 10. FIG. 2 illustrates the location of magneticallysignificant elements in magnetic read/write head 10 as they appear alongair bearing surface 14 of magnetic read/write head 10 of FIG. 1. In FIG.2, all spacing and insulating layers of magnetic read/write head 10 areomitted for clarity. Bottom shield 28 and bottom pole/top shield 24 arespaced to provide for a location of MR stack 30. A sense current iscaused to flow through MR stack 30 via bottom pole/top shield 24 andbottom shield 28. While the sense current is injected through the bottompole/top shield 24 and bottom shield 28 in FIGS. 1 and 2, otherconfigurations have MR stack 30 electrically isolated from bottompole/top shield 24 and bottom shield 28, with additional leads providingthe sense current to MR stack 30. As the sense current is passed throughMR stack 30, the read sensor exhibits a resistive response, whichresults in a varied output voltage. Because the sense current flowsperpendicular to the plane of MR stack 30, the reader portion ofmagnetic read/write head 10 is a current-perpendicular-to-plane (CPP)type device. Magnetic read/write head 10 is merely illustrative, andother CPP configurations may be used in accordance with the presentinvention.

FIG. 3 shows an ABS view of a typical tri-layer CPP MR sensor comprisingMR stack 50. MR stack 50 includes metal cap layer 52, first free layer54, nonmagnetic layer 56, second free layer 58, and metal seed layer 60.MR stack 50 is positioned between top shield 24 and bottom shield/lead28.

In operation, sense current I is passed through CPP MR stack 50. Sensecurrent I flows perpendicularly to the plane of the layers of the MRread sensor and experiences a resistance which is proportional to thecosine of an angle formed between the magnetization directions of thetwo free layers. The voltage across the CPP MR stack is then measured todetermine the change in resistance and the resulting signal is used torecover the encoded information from the magnetic medium. It should benoted that CPP MR stack 50 configuration is merely illustrative, andother layer configurations for CPP MR stack 50 may be used in accordancewith the present invention.

As described above, narrow reader widths are desired for retrieval ofdata stored on ultra-high density media having small areal size bits. AsMR sensors become smaller in size, particularly for sensors withdimensions less than about 0.1 micrometers (μm), the sensors have thepotential to exhibit an undesirable magnetic response to applied fieldsfrom the magnetic disc. One promising technique to reduce the effectivedimensions of MR sensors is to incorporate current confining paths, or“pinholes,” in a layer or layers of the MR stack. The current confiningpaths are formed such that they offer a path of lower resistance throughwhich the sense current flows. The sense current is thus confined to asmaller portion of the MR stack, thereby reducing the electrical profileof the MR sensor. Typically, these current confining paths are formedeither by etching a current confining path into a layer or layers of theMR stack, or by incorporating a layer of granular or porous materialinto the MR stack having naturally occurring current confining paths.Both of these techniques for incorporating current confining paths in anMR sensor are described in, for example, patent application Pub.2002/0051380 by Kamiguchi et al. The present invention is a morecontrollable approach to forming current confining paths (or, conductivenanoconstrictions) in an MR sensor which allows for increasedmagnetoresistance.

FIG. 4 a shows an ABS view and FIG. 4 b shows a top view of tri-layerCPP MR stack 100 according to an embodiment of the present invention.Similar to MR stack 50 shown in FIG. 3, MR stack 100 includes first freelayer 54, nonmagnetic layer 56, and second free layer 58. First freelayer 54, nonmagnetic layer 56, and second free layer 58 comprise themagnetically sensitive portion of MR stack 100. MR stack 100 furtherincludes layer of high resistivity material 102 formed on the top offirst free layer 54. MR stack 100 has a reader width and a reader stripeheight as shown in FIG. 4 b. The reader stripe height is typically setby lapping during the fabrication process. For clarity, FIGS. 4 a and 4b show only those layers necessary for the description of the presentembodiment. MR stack 100 typically includes additional layers and ispositioned between two electrodes to provide sense current I_(S),similar to the configuration of MR stack 50 in FIG. 3.

In order to facilitate the formation of conductive nanoconstrictions inMR stack 100, layer of high resistivity material 102 hasnanoconstriction precursor 110 formed therein. Various methods offorming nanoconstriction precursor 110 are described in detail below.Nanoconstriction precursor 110 is formed during wafer level fabricationat highly efficient region 115 of MR stack 100 (i.e., the area of MRstack 100 where first free layer 54, nonmagnetic layer 56, and secondfree layer 58 are most sensitive to magnetic field changes at themagnetic medium). In general, highly efficient region 115 of MR stack100 is located proximate to the ABS and generally centrally located withrespect to the reader width (as shown in FIG. 4 b).

After nanoconstriction precursor 110 is formed on MR stack 100, a punchcurrent, I_(p), is applied to MR stack 100. Punch current I_(p) isapplied via a contact or shield (such as electrodes/shields 24 or 28shown in FIG. 3) and typically has a magnitude of about 1-20 mA. Punchcurrent I_(p) is pulsed for a short amount of time, typically between0.1 ns and 100 ms, to form a conductive nanoconstriction through layerof high resistivity material 102. When punch current I_(p) is applied toMR stack 100, dielectric breakdown occurs at nanoconstriction precursor110 from the resulting high electric field. Dielectric breakdown voltageis a measure of the ability of an insulator to withstand a high electricfield stress without breaking down. When a critical electric field isexceeded, conduction paths, or nanoconstrictions, grow at microsecondspeeds through the insulator. The voltage necessary to cause dielectricbreakdown is based on the composition and thickness of layer of highresistivity material 102. A further discussion of dielectric breakdownand the formation of conductive nanoconstrictions is provided in B.Oliver, Q. He, X. Tang, and J. Nowak, J. Appl. Phys., Vol. 91, No. 7, p.4348 (2002), and is herein incorporated by reference.

Punch current I_(p) controls with nanometer precision the size of theconductive nanoconstriction. The conductive nanoconstriction istypically a metallic pinhole, and the size of the conductivenanoconstriction is adjustable based on the amplitude and duration ofpunch current I_(p) applied to nanoconstriction precursor 110. The widthor diameter of the conductive nanoconstriction is proportional to theamplitude and duration of punch current I_(p). Furthermore, the shape ofthe conductive nanoconstriction may be adjusted by adjusting thickness tof layer of high resistivity material 102.

During operation, the conductive nanoconstriction confines sense currentI_(S) (FIG. 3) to a much smaller and very efficient area of MR stack 100than in conventional designs. As a result, the effective reader widthand effective reader stripe height of MR stack 100 are much smaller,thereby increasing the efficiency and sensitivity of MR stack 100.Consequently, a greater change in resistance occurs in MR stack 100 asMR stack 100 passes over different data states on the magnetic medium,resulting in a greater voltage drop across MR stack 100 as sense currentI_(S) passes through it.

It should also be noted that the embodiment shown in FIGS. 4 a and 4 bis merely illustrative, and layer of high resistivity material 102 maybe formed anywhere within MR stack 100, depending on the desiredlocation of current confinement. For example, layer of high resistivitymaterial 102 may be formed between first free layer 54 and nonmagneticlayer 56, between nonmagnetic layer 56 and second free layer 58, orbeneath second free layer 58. Also, multiple layers of high resistivitymaterial including nanoconstriction precursors may be incorporated intoMR stack 100 (and punched with a punch current to form conductivenanoconstrictions) to allow for further confinement of sense currentI_(S). The multiple layers of high resistivity material may be formed ontop of one another, and at different locations throughout MR stack 100.

Nanoconstriction precursor 110 may be formed during wafer levelprocessing in a number of ways according to the present invention. Inone exemplary embodiment, layer of high resistivity material 102 is madeof an oxide material, such as oxide compounds of Ti, Al, and CoFe. Theoxide material has a non-uniform thickness such that the oxide materialis thinner within highly efficient region 115 of MR stack 100 thanoutside of highly efficient region 115. At the region of thinned oxidematerial, the dielectric breakdown voltage is much lower than outside ofthe thinned oxide material.

In another exemplary embodiment, layer of high resistivity material 102with a uniform thickness is formed on top of MR stack 100. Metal ionsare then implanted in layer of high resistivity material 102 withinhighly efficient region 115 to form nanoconstriction precursor 110. Theregion of implanted metal ions of nanoconstriction precursor 110 has alower dielectric breakdown voltage than the remainder of highresistivity material 102.

In a further exemplary embodiment, layer of high resistivity material102 with a uniform thickness is formed on top of MR stack 100. Anelectron beam is then applied to layer of high resistivity material 102within highly efficient region 115. At the location where the electronbeam is applied, the high resistivity material is transformed to a lowresistivity material, thereby forming nanoconstriction precursor 110.The low resistivity material at nanoconstriction precursor 110 has alower dielectric breakdown voltage than the remainder of highresistivity material 102.

In still another exemplary embodiment, layer of high resistivitymaterial 102 is an oxide material and is formed with a uniform thicknesson top of MR stack 100. A reactive ion etch is then performed on thelayer of high resistivity material 102 within highly efficient region115. The reactive ion beam must be focused and positioned with nanometerprecision. At the location where the reactive ion etch is performed, thehigh resistivity material is reduced to a pure metal. When a punchcurrent is applied, the pure metal at nanoconstriction precursor 110initiates a dielectric breakdown of high resistivity material 102 in thevicinity of nanoconstriction precursor 110.

FIG. 5 shows a perspective ABS view of tri-layer CPP MR stack 150according to another embodiment the present invention. Similar to MRstack 50 shown in FIG. 3, MR stack 150 includes first free layer 54,nonmagnetic layer 56, and second free layer 58. First free layer 54,nonmagnetic layer 56, and second free layer 58 comprise the magneticallysensitive portion of MR stack 150. MR stack 150 further includes oxidelayer 152 formed on the top of first free layer 54. MR stack 150 has areader width and a reader stripe height as shown. The reader stripeheight is typically set by lapping during the fabrication process. Forclarity, FIG. 5 shows only those layers necessary for the description ofthe present embodiment. MR stack 150 typically includes additionallayers and is positioned between two electrodes to provide sense currentI_(S), similar to the configuration of MR stack 50 in FIG. 3.

Oxide layer 152 is formed on first free layer 52 such that thinnedregion 155 has a smaller thickness than other portions of oxide layer152. Thinned region 155 is located at the most efficient region of MRstack 150, that is, at an area proximate to the ABS and generallycentrally located with respect to the reader width. This is the area ofMR stack 150 where first free layer 54, nonmagnetic layer 56, and secondfree layer 58 are most sensitive to magnetic field changes at themagnetic medium. The fabrication of MR stack 150 is subsequentlycompleted, resulting in a device including MR stack 150 positionedbetween two electrodes (similar to MR stack 50 positioned betweenshields/electrodes 24 and 28 in FIG. 3).

After MR stack 150 has been lapped to a desired stripe height, the ABSis covered by metal layer 160. Metal layer 160 is formed on the ABS suchthat, when MR stack 150 is positioned between the two electrodes in thecompleted reader, metal layer 160 forms a current path between theelectrode on the top of MR stack 150 and the electrode on the bottom ofMR stack 150. Subsequently, a punch current I_(p) is applied to MR stack150. Punch current I_(p) is applied via a contact or shield (such asshields 24 or 28 shown in FIG. 3) and typically has a magnitude of about1-20 mA. A portion of punch current I_(p), shown in FIG. 5 as precursorcurrent I_(PRE), is conducted through the top electrode to metal layer160 and through the bottom electrode. As the current flows through metallayer 160, it heats the ABS of MR stack 150. The largest power densitydissipation occurs in thinned region 155 near the ABS. As thinned region155 near the ABS heats, oxide material 152 in thinned region 155 istransformed from a high resistivity material to a low resistivitymaterial.

In this embodiment, metal layer 160 is a part of the nanoconstrictionprecursor according to the present invention, since metal layer 160initiates conductive nanoconstriction growth in oxide layer 152. Metallayer 160 is formed after wafer level fabrication on the ABS of MR stack150. That is, metal layer 160 is formed on the ABS after lapping of MRstack 150 to a desired reader stripe height. Thus, in this embodimentthe lapping step in wafer level fabrication is not a critical step inassuring that the nanoconstriction is formed in the highly efficientregion.

After oxide material 152 in thinned region 155 is transformed from ahigh resistivity material to a low resistivity material, a magnitude ofpunch current I_(p) applied to MR stack 150 is increased. Punch currentI_(p) is pulsed for a short amount of time, typically between 0.1 ns and100 ms, to form a conductive nanoconstriction through oxide layer 152.When punch current I_(p) is applied to MR stack 150, metal layer 160heats up and initiates dielectric breakdown in thinned region 155 nearthe ABS. The voltage necessary to cause dielectric breakdown is based onthe composition and thickness of oxide layer 152 and metal layer 160.Dielectric breakdown results in the formation of a pinhole, orconductive nanoconstriction, through oxide layer 152.

Punch current I_(p) controls with nanometer precision the size of theconductive nanoconstriction. The conductive nanoconstriction istypically a metallic pinhole, the size of which is adjustable based onthe amplitude and duration of punch current I_(p) applied to thinnedregion 155. The width or diameter of the conductive nanoconstriction isproportional to the amplitude and duration of punch current I_(p).Furthermore, the shape of the conductive nanoconstriction may beadjusted by adjusting thickness t of oxide layer 152 or metal layer 160.

During operation, the conductive nanoconstriction confines sense currentI_(s) (FIG. 3) to a much smaller and very efficient area of MR stack 150than in conventional designs. As a result, the effective reader widthand effective reader stripe height of MR stack 150 are much smaller,thereby increasing the efficiency and sensitivity of MR stack 150.Consequently, a greater change in resistance occurs in MR stack 150 asMR stack 150 passes over different data states on the magnetic medium,resulting in a greater voltage drop across MR stack 150 as sense currentI_(S) passes through it.

It should also be noted that the embodiment shown in FIG. 5 is merelyillustrative, and oxide layer 152 may be formed anywhere within MR stack150, depending on the desired location of current confinement. Forexample, oxide layer 152 may be formed between first free layer 54 andnonmagnetic layer 56, between nonmagnetic layer 56 and second free layer58, or beneath second free layer 58. Also, multiple layers of highresistivity material including thinned regions may be incorporated intoMR stack 150 (and punched with a punch current to form conductivenanoconstrictions) to allow for further confinement of sense currentI_(s). The multiple layers of high resistivity material may be formed ontop of one another, and at different locations throughout MR stack 150.

In summary, the present invention is an MR sensor having a decreasedelectrical profile due to a confining of the device sense current withina conductive nanoconstriction. The MR sensor includes a giantmagnetoresistive (GMR) stack and a layer of high resistivity material ona top of the GMR stack. The layer of high resistivity material includesa nanoconstriction precursor. When a punch current is applied at thenanoconstriction precursor, a conductive nanoconstriction is formedthrough the layer of high resistivity material at the nanoconstrictionprecursor. The width of the conductive nanoconstriction is adjustable byadjusting an amplitude and duration of the punch current. Furthermore,the shape of the conductive nanoconstriction is adjustable by adjustinga thickness of the layer of high resistivity material.

The embodiments heretofore described offer flexibility in the timing offormation of conductive nanoconstrictions in an MR stack. In theembodiment described in FIGS. 4 a and 4 b, the conductivenanoconstrictions are formed during wafer level processing, while in theembodiment described in FIG. 5, the conductive nanoconstrictions areformed at the bar or slider assembly level. In all embodiments, the useof a punch current allows for nanometer precision formation of theconductive nanoconstrictions, a feature which is important in themanufacture of contemporary highly efficient read heads.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. In particular, the MR sensor may take manydifferent forms in accordance with the present invention and is notlimited to the tri-layer configuration (two free layers with anonmagnetic spacer therebetween) heretofore described. For example, theMR sensor may include a multi-layered structure formed of a nonmagneticspacer layer positioned between a synthetic antiferromagnet (SAF) and afree layer. The magnetization of the SAF is fixed, typically normal tothe ABS of the MR sensor, while the magnetization of the free layerrotates freely in response to an external magnetic field. The SAFincludes a reference layer and a pinned layer which are magneticallycoupled by a coupling layer such that the magnetization direction of thereference layer is opposite to the magnetization of the pinned layer.

1. A current-perpendicular-to-the-plane (CPP) magnetoresistive (MR)sensor comprising: a giant magnetoresistive (GMR) stack; and a layer ofhigh resistivity material within the GMR stack, the layer of highresistivity material including a nanoconstriction precursor formed suchthat, when a punch current is applied at the nanoconstriction precursor,a conductive nanoconstriction is formed through the layer of highresistivity material at the nanoconstriction precursor.
 2. The MR sensorof claim 1, wherein the nanoconstriction precursor comprises a thinnedregion in the layer of high resistivity material.
 3. The MR sensor ofclaim 1, wherein the nanoconstriction precursor comprises a region inthe layer of high resistivity material which has been implanted withmetal ions by an ion beam.
 4. The MR sensor of claim 1, wherein thenanoconstriction precursor comprises a region in the layer of highresistivity material which has been transformed to a low resistivitymaterial by an electron beam.
 5. The MR sensor of claim 1, wherein thenanoconstriction precursor comprises a region in the layer of highresistivity material which has been reduced to a metal via a reactiveion etch.
 6. The MR sensor of claim 1, wherein a width of the conductivenanoconstriction is adjustable by adjusting an amplitude and duration ofthe punch current.
 7. The MR sensor of claim 1, wherein a shape of theconductive nanoconstriction is adjustable by adjusting a thicknessprofile of the layer of high resistivity material.
 8. The MR sensor ofclaim 1, wherein the layer of high resistivity material comprises anoxide material.
 9. The MR sensor of claim 8, wherein the oxide materialis selected from the group consisting of oxide compounds of Ti, Al, andCoFe.
 10. A method of making a current-perpendicular-to-the-plane (CPP)magnetoresistive (MR) sensor comprising: forming a giantmagnetoresistive (GMR) stack including a layer of high resistivitymaterial; forming a nanoconstriction precursor in the layer of highresistivity material; and applying a punch current to form a conductivenanoconstriction through the layer of high resistivity material at thenanoconstriction precursor.
 11. The method of claim 10, wherein forminga nanoconstriction precursor comprises thinning a region in the layer ofhigh resistivity material.
 12. The method of claim 10, wherein forming ananoconstriction precursor comprises: coating an air bearing surface ofthe GMR stack with a thin metal layer; and applying a precursor currentto the GMR stack wherein the thin metal layer conducts the precursorcurrent and wherein the precursor current conducted by the thin metallayer heats a thinned region of the layer of high resistivity material.13. The method of claim 10, wherein forming a nanoconstriction precursorcomprises implanting metal ions at a region of the layer of highresistivity material with an ion beam.
 14. The method of claim 10,wherein forming a nanoconstriction precursor comprises transforming aregion of the layer of high resistivity material to a low resistivitymaterial with an electron beam.
 15. The method of claim 10, whereinforming a nanoconstriction precursor comprises transforming a region ofthe layer of high resistivity material to a metal with a reactive ionetch.
 16. The method of claim 10, wherein forming a nanoconstrictionprecursor in the layer of high resistivity material comprises: selectingan area of high reader efficiency in the GMR stack; and forming ananoconstriction precursor in the layer of high resistivity material atthe area of high reader efficiency.
 17. The method of claim 16, whereinthe area of high reader efficiency is located proximal to an air bearingsurface of the GMR stack.
 18. The method of claim 10, furthercomprising: adjusting a thickness profile of the layer of highresistivity material to adjust a shape of the conductivenanoconstriction.
 19. The method of claim 10, further comprising:adjusting an amplitude and duration of the punch current to adjust awidth of the conductive nanoconstriction.
 20. The method of claim 10,wherein the layer of high resistivity material comprises an oxidematerial.
 21. The method of claim 20, wherein the oxide material isselected from the group consisting of oxide compounds of Ti, Al, andCoFe.
 22. The method of claim 10, wherein the layer of high resistivitymaterial has a thickness of less than 6 Å.
 23. The method of claim 10,wherein the GMR stack includes two magnetic free layers separated by anonmagnetic layer.
 24. The method of claim 23, wherein the nonmagneticlayer comprises a metal.
 25. A method of forming a conductivenanoconstriction in a layer of high resistivity material, the methodcomprising: forming a nanoconstriction precursor in the layer of highresistivity material; and applying a punch current to form a conductivenanoconstriction through the layer of high resistivity material at thenanoconstriction precursor.
 26. The method of claim 25, wherein forminga nanoconstriction precursor comprises thinning a region in the layer ofhigh resistivity material.
 27. The method of claim 25, wherein forming ananoconstriction precursor comprises implanting metal ions at a regionof the layer of high resistivity material with an ion beam.
 28. Themethod of claim 25, wherein forming a nanoconstriction precursorcomprises transforming a region of the layer of high resistivitymaterial to a low resistivity material with an electron beam.
 29. Themethod of claim 25, wherein forming a nanoconstriction precursorcomprises transforming a region of the layer of high resistivitymaterial to a metal with a reactive ion etch.
 30. The method of claim25, further comprising: adjusting an amplitude and duration of the punchcurrent to adjust a width of the conductive nanoconstriction.
 31. Themethod of claim 25, further comprising: adjusting a thickness profile ofthe layer of high resistivity material to adjust a shape of theconductive nanoconstriction.